How It Works
Darcy-Weisbach Equation
The Darcy-Weisbach equation is the fundamental formula for calculating pressure drop due to friction in pipe flow:
dP = f * (L/D) * (rho * V^2 / 2)
Where: dP = pressure drop, f = Darcy friction factor, L = pipe length, D = inside diameter, rho = fluid density, V = flow velocity
Reynolds Number
The Reynolds number determines the flow regime and is essential for calculating the friction factor:
Re = (V * D) / v
Where: V = velocity, D = diameter, v = kinematic viscosity (cSt * 10^-6)
- Laminar (Re < 2,300): Smooth, parallel flow layers. f = 64/Re
- Transitional (2,300 < Re < 4,000): Unstable, unpredictable flow
- Turbulent (Re > 4,000): Chaotic flow with eddies. Use Colebrook equation
Friction Factor (Moody)
For laminar flow: f = 64 / Re
For turbulent flow, the Colebrook-White equation is used (solved iteratively):
1/sqrt(f) = -2*log10(e/3.7D + 2.51/(Re*sqrt(f)))
For smooth hydraulic hose, the relative roughness (e/D) is typically 0.000005 to 0.00001.
Key Factors Affecting Pressure Drop
- Velocity: Pressure drop increases with velocity SQUARED
- Viscosity: Higher viscosity = higher friction (especially in laminar flow)
- Length: Pressure drop is directly proportional to length
- Diameter: Pressure drop is inversely proportional to D^5
Pressure Loss Calculator
Calculate pressure drop in hydraulic lines using Darcy-Weisbach equation with Moody friction factor.
Fluid Properties
Pressure Loss Results
Hydraulic Fluid Properties
| Fluid | Viscosity @ 40C | Viscosity @ 100C | Density |
|---|---|---|---|
| ISO VG 32 | 32 cSt | 5.4 cSt | 0.87 kg/L |
| ISO VG 46 | 46 cSt | 6.8 cSt | 0.87 kg/L |
| ISO VG 68 | 68 cSt | 8.7 cSt | 0.88 kg/L |
| ATF (Dexron) | 35 cSt | 7.5 cSt | 0.85 kg/L |
| Water | 0.66 cSt | 0.29 cSt | 1.0 kg/L |
Note: Viscosity varies significantly with temperature. Use actual operating temperature viscosity for accurate results.